Development is currently underway for communication systems that perform wireless data transmission over short distances, such as intra-body communication and non-contact communication. Characteristically, a communication system of this type is capable of conducting wireless communication in a simple manner, and has a very low risk of being intercepted, for example.
Particularly, in intra-body communication, a human body is used as a communication path medium, and accordingly, communication can be conducted with an easy-to-understand interface that is to be touched by a user. Also, intra-body communication is designed for communication in the vicinity of a human body. Therefore, there is no need to transmit radio waves over a long distance, while there is the advantage that power consumption is low.
Conventional intra-body communication is roughly classified into two methods: an electric current method for transmitting a current signal in a human body, and an electric field method for transmitting a signal by applying an electric field to a human body. By the former electric current method, a human body needs to be in contact with the respective electrodes of a transmitter and a receiver. Therefore, there are drawbacks that the user feels uncomfortable, and communication becomes unstable when the contact state between the skin and the electrodes changes due to perspiration or the like. In the case of the latter electric field method, even when a human body is not in contact with the electrodes of the transmitter and the receiver, an electric field can be applied to the human body, and communication can be conducted. Accordingly, this method has a wider range of application.
For example, there is a suggested intra-body communication system in which electric fields are generated almost only between the electrodes of a first communication device and a second communication device, and the possibility that intra-body communication will be started against the user's will is lowered (see Patent Document 1, for example).
There is also a suggested intra-body communication system in which an electronic device has a human-body electrode and an external electrode, and the impedance between the human-body electrode and the external electrode is maximized at the frequency to be used for field coupling. This intra-body communication system is hardly affected by static electricity even when electrostatically charged, and can maintain satisfactory performance (see Patent Document 2, for example).
The applicant is of the opinion that, so as to further improve quality in an intra-body communication system of the electric field type, field coupling needs to be realized with higher efficiency even when a human body is interposed between the electrodes of the transmitter and the receiver. Also, where field coupling between electrodes via a human body is stronger, there is the advantage that the electrodes of the transmitter and the receiver can be made smaller in size.
The idea that high-efficiency field coupling is required and devices can be made smaller in size by making field coupling stronger applies not only to intra-body communication but also to non-contact communication systems and wireless power transmission systems of the field coupling type.